1. Field of the Invention
The present invention relates to a microcomputer which can access an external memory, and particularly relates to a single-chip microcomputer for which the boundary address of the memory mapping boundary between internal and external memories can be set by software programming.
2. Description of the Prior Art
With application products using single-chip microcomputers becoming more sophisticated recently, the single-chip microcomputer itself is increasingly required to have higher capabilities with much greater program size.
A single-chip microcomputer usually has an incorporated memory inside, where a program is written during diffusion process, so that the microcomputer executes processing according to the program written there. Because of an increase in program size and data amount to meet the requirement for higher function and performance, a recent single-chip microcomputer is often provided with an additional memory storing programs and data connected outside. In this case, the incorporated memory (internal memory) is mapped to a certain address region in the memory space and the memory connected outside (external memory) is mapped to an address region different from the one where the internal memory is mapped. They can be accessed with specification of the mapped address. The internal and external memory spaces are generally on both sides of their boundary address and mapped so as to have continuous addresses.
In a conventional single-chip computer, the boundary address between the space for internal memory mapping and the space for external memory mapping is fixed. This means that the maximum accessible memory space cannot be changed from the size obtained by the internal memory capacity and the external memory capacity. In other words, the memory space cannot be easily extended in response to increase in program size and data amount.